Important human health concerns, including: cancer, bacterial and viral infection, atherosclerosis, and neurodegenerative diseases, all to various degrees, involve components of the endosomal network. The functions of the endosomal network are, in part, dependent on microtubules. Our insufficient understanding of the endosomal and microtubule network has been obtained, partially, through the study of their interactions with small molecules. Considering the value of small molecules as probes of biological function, this research project outlines the use of new probes derived from biologically active, and medicinally important, natural products. Our immediate goal is t further elucidate the mechanism of action of several biologically active small molecules that interact with microtubules, endosomal, and other protein receptors. These mechanistic clarifications will include the identification and characterization of new small molecule receptors, as well as more detailed studies of known small molecule/receptor interactions. Details of these interactions should provide new targets for selective chemotherapeutic interventions. For example, ilimaquinone ('lima-quinone'), an antimitotic, anti-HIV, sesquiterpenoid quinone, is reported to have profound effects on endosomal trafficking. The biologically relevant target of this natural product will be pursued using active analogs of ilimaquinone. In a related manner, avarone, a structurally similar natural product, also with significant antimitotic activity, will be modified such that its receptor can be identified and characterized. Agents that inhibit specific endosomal events could be used to control intracellular trafficking of cholesterol or beta-amyloid; or possibly inhibit viral entry into the cell. In addition, to better explain the mechanism of action of important antimitotic agents a search for small molecule mediated interactions with microtubules will be initiated. Novel binding assays will examine both the inhibition of, and the promotion of, new protein-microtubule interactions in the presence of several antimitotic agents, including ilimaquinone, avarone, and the anticancer drug taxol. Protein co- receptors found for these antimitotic agents will be identified and characterized. Furthermore, to study known and unknown interactions, a combinatorial search of small molecule binding to tubulin will be undertaken. Identification and study of interactions of the endosomal and microtubule network will provide new opportunities for designing agents with highly specific interactions. Inhibition of specific microtubule or endosomal activities with these agents could represent new strategies for controlling a number of diseases.